The integrity of cast-in-place foundation piles is a major concern in geotechnical engineering. In this study, distributed fibre optic sensing (DFOS) cables, embedded in a pile during concreting, are used to measure the changes in concrete curing temperature profile to infer concrete cover thickness through modelling of heat transfer processes within the concrete and adjacent ground. A field trial was conducted at a high-rise building construction site in London during the construction of a 51 m long test pile. DFOS cables were attached to the reinforcement cage of the pile at four different axial directions to obtain distributed temperature change data along the pile. The monitoring data shows a clear development of concrete hydration temperature with time and the pattern of the change varies due to small changes in concrete cover. A one-dimensional axisymmetric heat transfer finite element (FE) model is used to estimate the pile geometry with depth by back analysing the DFOS data. The results show that the estimated pile diameter varies with depth in the range between 1.40 and 1.56 m for this instrumented pile. This average pile diameter profile compares well to that obtained with the standard Thermal Integrity Profiling (TIP) method. A parametric study is conducted to examine the sensitivity of concrete and soil thermal properties on estimating the pile geometry.

The design of a 60-storey tall residential tower with a two-level basement in east London has had to take account of two London Underground tunnels which pass beneath the centre of the site, resulting in ‘working’ pile loads of up to 45 MN. The foundation solution comprised a stiff raft with cast in situ bearing piles up to 2·4 m dia. and 61 m long founding in the grade A/B chalk stratum. Some piles, using bentonite support fluid, needed to be constructed within the tunnel exclusion zones, which required a concession from London Underground. This paper presents the Osterberg-cell preliminary pile load test and results, foundation design, working pile construction and limitations on piles constructed in the London Underground exclusion zones. The paper also discusses the control on the bentonite support fluid properties and monitoring of its properties during pile construction.